Re: [PATCH v5 4/9] mm/demotion: Build demotion targets based on explicit memory tiers

[Aneesh Kumar K V]

On 6/8/22 4:21 AM, Tim Chen wrote:
> On Fri, 2022-06-03 at 19:12 +0530, Aneesh Kumar K.V wrote:
> > +int next_demotion_node(int node)
> > +{
> > +	struct demotion_nodes *nd;
> > +	int target, nnodes, i;
> > +
> > +	if (!node_demotion)
> > +		return NUMA_NO_NODE;
> > +
> > +	nd = &node_demotion[node];
> > +
> > +	/*
> > +	 * node_demotion[] is updated without excluding this
> > +	 * function from running.
> > +	 *
> > +	 * Make sure to use RCU over entire code blocks if
> > +	 * node_demotion[] reads need to be consistent.
> > +	 */
> > +	rcu_read_lock();
> > +
> > +	nnodes = nodes_weight(nd->preferred);
> > +	if (!nnodes)
> > +		return NUMA_NO_NODE;
> > +
> > +	/*
> > +	 * If there are multiple target nodes, just select one
> > +	 * target node randomly.
> > +	 *
> > +	 * In addition, we can also use round-robin to select
> > +	 * target node, but we should introduce another variable
> > +	 * for node_demotion[] to record last selected target node,
> > +	 * that may cause cache ping-pong due to the changing of
> > +	 * last target node. Or introducing per-cpu data to avoid
> > +	 * caching issue, which seems more complicated. So selecting
> > +	 * target node randomly seems better until now.
> > +	 */
> > +	nnodes = get_random_int() % nnodes;
> > +	target = first_node(nd->preferred);
> > +	for (i = 0; i < nnodes; i++)
> > +		target = next_node(target, nd->preferred);
>
> We can simplify the above 4 lines.
>
> 	target = node_random(nd->preferred);
>
> There's still a loop overhead though :(

Will fix in next update.

> > +
> > +	rcu_read_unlock();
> > +
> > +	return target;
> > +}
> > +

-aneesh